Crystal grower

ABSTRACT

A crystal grower furnace assembly having a gastight housing, a crystal pulling rod extending into the housing coaxially with the vertical axis of the housing and the heating elements. The housing is divided into a first chamber adapted for growing a crystal by the Czochralski technique, from a melt of the crystalline material, a second chamber above the first chamber having a port for evacuation and admission of gases, and a narrow neck connecting the chambers in coaxial alignment. The rod extends through the neck and is adapted for rotational and vertical movement therein. One heater element, situated near the juncture of the neck portion and the second chamber, is employed to heat boron oxide to a liquid which is used to seal the space between the rod and the inner wall of the neck portion to seal off the first chamber from the second chamber during the crystal growth operation. The second heater is situated near the top of the first chamber and is employed to prevent the condensation of arsenic during crystal growth.

United States Patent 1191 Allred Dec. 31, 1974 1 CRYSTAL GROWER Worth P. Allred, West Covina, Calif.

[73] Assignee: University of Southern California,

Los Angeles, Calif.

22 Filed: Feb. 5, 1973 21 Appl.No.:329,746

[75] Inventor:

[52] US. Cl. 23/273 SP, 23/301 SP [51] Int. Cl B0lj 17/06, BOIj 17/18 [58] Field of Search 23/273 SP, 301 SP, 301 R,

Primary Examiner-Norman Yudkoff Assistant Examiner-D. Sanders Attorney, Agent, or Firm-Harris, Kern, Wallen & Tinsley H/GH TEMPERA TURE-\3/ 2 mum SEA L ENT HEATER ASSEMBLY [5 7 ABSTRACT A crystal grower furnace assembly having a gastight housing, a crystal pulling rod extending into the housing coaxially with the vertical axis of the housing and the heating elements. The housing is divided into a first chamber adapted for growing a crystal by the Czochralski technique, from a melt of the crystalline material, a second chamber above the first chamber having a port for evacuation and admission of gases, and a narrow neck connecting the chambers in coaxial alignment. The rod extends through the neck and is adapted for rotational and vertical movement therein. One heater element, situated near the juncture of the neck portion and the second chamber, is employed to heat boron oxide to a liquid which is used to seal the space between the rod and the inner wall of the neck portion to seal off the first chamber from the second chamber during the crystal growth operation. The second heater is situated near the top of the first chamber and is employed to prevent the condensation of arsenic during crystal growth.

5 Claims, 1 Drawing Figure HEA TEE 71 SSEMBL y CRYSTAL GROWER BACKGROUND OF THE INVENTION The present inventiondsdirected to a furnace assembly for the growth of single crystals having a volatile component by the Czochralski technique and to a method of preparing such crystals.

In particular, the invention is directedto an apparatus for the preparation of single crystals of gallium arsenide and indium arsenide of predetermined composition. These crystals are used in the electronics industries for light emitting diodes and high frequency devices. These crystals are also used for infrared window.

There are two methods used for the growth of single crystals of compounds having one or more volatile components. The first method is referred to as the horizontal Bridgman technique. In the Bridgman technique, a sealed, horizontal'furnace is employed which has no moving parts within the crystal growing chamber of the furnace. The crystal is grown from a melt in a boat, such as a quartz boat. The melt is directionally cooled along the longitudinal axis of the boat to get a single crystal. The Bridgman technique requires relatively expensive equipment and requires a certain expertise inoperation. The second method is the vertical pull technique, commonly referred'to as the crystal pull or Czochralski technique. In this technique, the crystal is grown by inserting'a seed crystal into a melt of the raw material and slowlywithdrawingthe seed vertically upward from the melt while rotating the seed and the growing crystal. The conventional apparatus employed in the Czochralski technique is not suitable for the growth of crystals with volatile components. The growth of arsenide compound crystals of uniform and predetermined composition by the (Izochralski technique has proven to be difficult and'quite costly. At the melt temperature for most arsenide compounds, the vapor pressure of arsenic approaches atmospheric pressure. Thus, when employing the conventional apparatus employed in the Czochralski technique, the arsenic is readily lost from the melt to the cold walls of the system.

There have beenseveral attempts in the art to circumvent the above problems. These attempts have been partially successful, but generally the attempts have created other problems which are just as serious.

U.S. Pat. No. 3,074,785 discloses an apparatus for the preparation of crystals from. a melt of a multicomponent material having a volatile component. The apparatus disclosed in this patent employs magnets to support and rotate the seed rod' which is sealed within a quartz tube (see FIG. 1 of US. Pat. No. 3,074,785). Due to the lack of mechanical drive, through the quartz tube, the entire tube can be heated to a temperature in excess of 610C. without the loss of arsenic vapor or other volatile components. Although this apparatus solves one problem, it has created another. The employment of magnets for pulling and rotating the rod has created serious problems in crystal growth. It has been found that the rotation is jerky and the pull rate is not uniform; these characeristics are due to the spongy action of the magnetic coupling and the friction of the bearings. Because of thisjerky rotational motion and nonuniform pull rate, it is difficult to obtain high quality crystals without dislocations. In addition to this serious problem, the apparatus of the patent has been found to be costly and difficult to load and to clean.

Another apparatus is described by J. L. Richards in the Journal ofScientiflc Instrumentation, Vol. 34, pages 289-290 (1957). This system employs a gallium seal, similar to a conventional mercuryseal used in stirrers for chemical apparatus, to contain the arsenic vapor. Richards was able to grow small single crystals employ ing this apparatus. However, this apparatus has never been used successfully because it has several problems associated with its construction and operation. The apparatus is complicated and expensive to construct. The arsenic vapors react with the gallium in the seal causing the gallium to become a solid which interferes with the rotation of the seed rod. In addition, the arsenic vapor is continually lost to the gallium in the seal, thus causing a constant change in the composition of the crystal being grown from the melt.

Another method is disclosed by E. J. Metz in the Journal ofApplied Physics, Vol. 33, page 2016 (1962), and J. B. Mullin in the Journal ofPhysics and Chemistry of Solids, Vol. 26, page 782 (1965). These two papers disclose an apparatus for the preparation of lead telluride, lead selenide and gallium arsenide. The apparatus disclosed in these papers employs a boron oxide covering over the melt of the crystal raw material. In the case of gallium arsenide, the boron oxide prevents the escape of arsenic vapors from the raw material. Although this apparatus permits the growth of crystals having continuity in chemical composition, the crystals can be contaminated since they come in direct contact with the-boron oxide, and the crystal protruding through the boron oxide is encapsulated in the boron oxide as it is pulled from the melt which causes stress in the crystal. In addition, it has been found difficult to react gallium and arsenic under the boron oxide shield. This has necessitated the use of prereacted starting material.

SUMMARY OF THE INVENTION The invention is directed to an apparatus used in the preparation and growth ofa single crystal from a multicomponent material having a highly volatile component.

In particular, the furnace assembly of the present invention is employed for the growth of an uncontaminated arsenide compound crystal of predetermined and uniform composition. The apparatus employs the Czochralski pull technique of crystal growth using a rotating rod having a crystal seed attached to the lower end which is slowlywithdrawn upwardly from a melt of the raw material. The apparatus has a crystal growing chamber and an upper neck portion through which the rod can be rotated and withdrawn. An inert, high temperature liquid sealant is used in the neck portion to seal off the crystal growing chamber from the outer environment to prevent the loss of arsenic from the melt during the crystal growing process.

It is an object of the present invention to provide an apparatus and method for producing a single crystal of a multicomponent composition having a volatile component. More particularly, it is an object to provide an apparatus and method for the production of a single crystal of an arsenide compound having a predetermined and uniform composition.

Another object of the present invention is to provide an apparatus for growing a single crystal of a multicomponent composition, having a volatile component, by the Czochralski pull technique employing an externally driven crystal growing rod for a smooth rotational and -nent compositions from unreacted components.

DESCRIPTION OF THE DRAWING The drawing illustrates a vertical sectional view of the apparatus of this invention.

DETAILED DESCRIPTION OF THE INVENTION Crystal growing furnace assembly of this invention includes an envelope 11 having a first chamber 12, a second chamber 13 and a neck portion 14 interconnect ing the two chambers along the vertical axis of the furnace assembly. The envelope is divided into an upper section 11a and a bottom section 11b. Within the first chamber 12 there is situated a crucible 16 for the melt 17. The crucible 16 is supported by stand l8 which rests on the floor 19 of the first chamber 12. The second chamber 13 has a'conduit 20 connected to a three-way valve 21 which is connected to a conventional vacuum assembly for evacuation of gas or gases from the furnace assembly and to a gas supply for the admission of gas or gases into the furnace assembly. The upper end of the second chamber 13, which is openended, is sealed off with a cover plate 22. The bottom portion of the cover plate 22 is sealed to the upper edge 23 of the envelope 11 with a vacuum seal, such as an O-ring type seal 24. The cover 22 has a circular aperture 25 which is substantially coaxial with the vertical axis of the furnace assembly 10.

The top portion of the first chamber is interconnected to the bottom portion of the second chamber by the neck 14. The neck 14 has an internal bore 27 defined by an inner wall 29 of the neck. A cylindrical rod 28 extends through the aperture 25, second chamber 13, and bore 27 into the first chamber 12. The rod 28 is coaxial with the vertical axis of the furnace assembly 10. The diameter of the rod 28 is less than the diameter of the aperture 25 and bore 27. The rod is adapted for rotation and vertical movement within the aperture 25 and bore 27. The aperture 25 and the rod 28 are sealed with a conventional O-ring seal 26 and packed with vacuum grease.

The crystal grower furnace assembly 10 is illustrated in its operational mode. The space between the rod 28 and the inner wall 29 of the neck 14 is sealed off with an inert, high'temperature liquid sealant 31. In the preferred embodiment of the present invention, the sealant 31 is hot liquid boron oxide. A heater assembly 32 is situated externally about the neck 14 to heat and maintain the sealant 31 in the liquid state during the crystal growth operation. A crystal seed 34 is attached to the bottom end of the rod 28 with a conventional seed holder 35 such as Tantalum. Tungsten or Molybdenum. An all quartz seed holder can also be used. The crystal seed 34 serves as the nucleus for the growing crystal 36 which is pulled from the melt 17. The melt is maintained at the proper temperature by an induction heater assembly 38 which is located outside of the envelope 1] in the horizontal plane of crucible 16.Another heater element 39 is positioned externally about the first chamber 12 above the horizontal plane ofcrucible 16. Heater assembly 39 maintains the temperature in the upper portion of the first chamber 12 at a predetermined value to prevent the condensation of arsenic due to the cold walls of the chamber. Operation of the Crystal Grower Furnace Assembly The crystal growth procedure is initiated by charging the crucible 16 with the raw materialfor the crystal. For the preparation of gallium a'rsenide, pure gallium and pure arsenic can be added to the crucible. In the case of indium arsenide, indium and arsenic are added to the crucible. Naturally, prereacted components can also be added to the crucible. The seed crystal 34 is aligned and secured to the end of the rod 28 by any suitable means, such as Tantalum or Tungsten wire or by an all quartz seed holder. Solid boron oxide glass is placed in the second chamber 13; the solid does not seal the neck. The assembly 10 is then evacuated by means ofa conventional vacuum assembly (not shown) connected to valve 21. When the desired vacuum has been achieved, heating element 32 is then activated to a sufficient temperature to melt the solid boron oxide glass. After the boron oxide has melted, helium gas or another comparable inert gas is admitted into the second chamber 13 via valve 21 to force the liquid boron oxide down into the space between the rod 28 and the inner wall 29 of the neck 14. l have found that helium under atmospheric pressure is-normall y sufficient to force the molten boron oxide into the neck portion. The liquid boron oxide creates a liquid seal between the two chambers 12 and 13, which does not interfere with rotation orvertical movement of the rod 28. The boron oxide seal 31 prevents the loss of the volatile component from the melt' 17. The boron oxide is inert to arsenic and very little arsenic vapor is absorbed into the liquid boron oxide.

Induction heater assembly 38 is next actuated to melt the charge in the crucible 16. The heater 39 is also actuated to heat the upper portion ofthe first chamber 12 to prevent condensation of arsenic on the quartz walls. When the melt 17 has been brought to growth temperature this'temperature can be measured with a pyrometer the rod 28 is lowered until the seed crystal 34 comes in contact with the upper surface of the melt 17. The rod is then slowly withdrawn upwardly at a rate of about one inch per hour while being rotated at a predetermined rate to commence the growth of a single crystal from the melt.

The rod is connected directly to a mechanical. drive which both rotates the rod and vertically raises it at a predetermined rate.' The direct mechanical drive eliminates vibration and'jerky or modulated motions common to magnetic coupling. A conventional mechanical drive is employed in this invention. Suitable drives can be obtained from Futurecraft Corp., 15430 E. Proctor Avenue, City of Industry, California. See also the drives described in the following U.S. Pat. Nos. 2,793,103;

2,927,008; 3,198,606; and 3,488,l57.

one mole (69.72 grams) of gallium and one mole (74.92 grams) of arsenic. The volume of the second chamber 12 is about 500 cc. A sufficient amount of arsenic is added to the melt charge to insure that the melt has a predetermined stoichiometric composition at about 610C. (a measurable amount of arsenic escapes from the melt as vapor into the second chamber). The crucible is placed on stand 18 located in the lower section 11b of quartz envelope 11. The crystal growing rod 28 is positioned in the upper section 11a of the quartz envelope 11. The crystal seed 34 is attached to the lower end of the rod 28. The lower section 1111 is welded to the upper section 11a to form envelope 11. Solid boron oxide glass (about four grams) is placed in the second chamber 13 of the envelope 11. The rod 28 is passed through the aperture 25 of the cover plate 22, and the cover plate is placed over the open end of the second chamber 13 and sealed thereto employing an O-ring 24 and vacuum grease such as an Apiezon Brand vacuum grease.

After the envelope 11 has been evacuated to a pressure of less than 0.01 Torr, the heater 32 situated about the neck 14 is heated to a temperature of about 6l0C. to melt the solid boron oxide glass. After the boron oxide has melted and has been degassed, helium gas is allowed to enter into the second chamber 13 via port at a positive pressure of about 1 atmosphere which forces the hot liquid boron oxide down into the space between the rod 28 and the inner wall 29 of the neck 14. The diameter of the inner wall 29 is about 1 mm. greater than the diameter of the rod 28. The pressure is controlled so that about two-thirds of the upper portion of the bore 27 is filled with the liquid sealant 31.

The induction heater assembly 38 about the lower portion of chamber 12 is actuated to heat the melt 17 to a temperature of about 600C. Simultaneously, the heater 39 is actuated to heat the top portion of the first chamber 12 to a temperature of about 600C. When the melt 17 has reached a temperature of 600C. (as measured by a pyrometer), the rod 28 is lowered so that the seed 34 touches the top surface of the melt 17. The rod 28 is rotated at about rpm. and withdrawn vertically upward at a rate of about 1 inch per hour.

The above method can also be used to prepare indium arsenide by employing a charge of 114.76 grams of indium and 74.9 grams of arsenic.

I claim as my invention:

1. A furnace assembly for the growth of an uncontaminated arsenide compound crystal of predetermined composition, including:

a gas-tight housing, said housing having a first chamber adapted for the growth of crystals from a melt by the crystal pull technique, a second chamber and a neck interconnecting the two chambers along the vertical axis of said housing, said first chamber adapted to support a melt of the arsenide compound for crystal growth, said second chamber having a circular aperture coaxial with said axis and a port for the evacuation and admission of gas out of and into said housing, said neck having a circular bore coaxial with said axis and communicating with said chambers;

a cylindrical rod passing through said aperture into said housing coaxial with said axis, the lower end of said rod extending into said first chamber, said rodjournaled in said bore and adapted for rotation about and vertical movement along said axis for the growth of a crystal from the melt;

means external of said housing for rotating and vertically moving said rod for crystal growth;

inert sealant means for sealing said first chamber from said second chamber, said means being stored as a solid at ambient temperatures in said second chamber prior to the operation of said assembly, and said means sealing the space between said rod and the wall of said bore as a liquid at elevated temperatures, said means adapted to seal said space at elevated temperatures while said rod is in motion;

means external of said housing for heating said sealant means; and

means external of said housing for heating the melt of the arsenide compound.

2. The furnace assembly as defined in claim 1 including a heating means external of said housing for maintaining the crystal grown from the melt at a predetermined annealing temperature in said first chamber.

3. The furnace assembly as defined in claim 1 wherein said inert sealant means is boron oxide.

4. The furnace assembly as defined in claim 1 includ ing means for holding the melt in said first chamber.

5. A furnace assembly for the growth of an uncontaminated arsenide compound crystal of predetermined composition, including:

a gas-tight quartz housing, said housing having a first chamber adapted for the growth of crystals from a melt by the crystal pull technique, a second cham ber and a neck interconnecting the two chambers along the vertical axis of said housing, said first chamber adapted to support a melt of the arsenide compound for crystal growth, said second chamber having a circular aperture coaxial with said axis and a port for the evacuation and admission of gas out of and into said housing, and said neck having a circular bore coaxial with said axis communicating with said chamber;

a cylindrical rod passing through said aperture into said housing coaxial with said axis, said rod journaled into said bore and adapted for rotation about and vertical movement along said axis in said housing for the growth of a crystal from the melt;

means external of said housing for rotating and vertically moving said rod for crystal growth;

a predetermined amount of boron oxide in said housing, said boron oxide being stored as a solid at ambient temperature in said second chamber prior to the operation of said assembly, and said boron oxide sealing the space between said rod and the internal wall of said bore as a liquid at elevated temperatures;

means external of said housing for heating and melting said boron oxide;

means external of said housing for heating the melt of the arsenide compound; and

means external of said housing for maintaining the crystal grown from the melt at a predetermined annealing temperature in said first chamber. 

1. A FURNACE ASSEMBLY FOR THE GROWTH OF AN UNCONTAMINATED ARSENIDE COMPOUND CRYSTAL OF PREDETR ERMINED COMPOSITION INCLUDING: A GAS-TIGHT HOUSING, SAID HOUSING HAVING A FIRST CHAMBER ADAPTED FOR THE GROWTH OF CRYSTALS FROM A MELT BY THE CRYSTAL PULL TECHNIQUE, A SECOND CHAMBER AND A NECK INTERCONNECTING THE TWO CHAMBERS ALONG THE VERTICAL AXIS OF SAID HOUSING, SAID FIRST CHAMBER ADAPTED TO SUPPORT A MELT OF THE ARSENIDE COMPOUND FOR CRYSTAL GROWTH, SAID SECOND CHAMBER HAVING A CIRCULAR APERTURE COAXIAL WITH SAID AXIS AND A PORT FOR THE EVACUATION AND ADMISSION OF GAS OUT OF AND INTO SAID HOUSING, SAID NECK HAVING A CIRCULAR BORE COAXIAL WITH SAID AXIS AND COMMUNICATING WITH SAID CHAMBERS; A CYLINDRICAL ROD PASSING THROUGH SAID APERTURE INTO SAID HOUSING COAXIAL WITH SAID AXIS, THE LOWER END OF SAID ROD EXTENDING INTO SAID FIRST CHAMBER, SAID ROD JOURNALED IN SAID BORE AND ADAPTED FOR ROTATION ABOUT AND VERTICAL MOVEMENT ALONG SAID AXIS FOR THE GROWTH OF A CRYSTAL FROM THE MELT;
 2. The furnace assembly as defined in claim 1 including a heating means external of said housing for maintaining the crystal grown from the melt at a predetermined annealing temperature in said first chamber.
 3. The furnace assembly as defined in claim 1 wherein said inert sealant means is boron oxide.
 4. The furnace assembly as defined in claim 1 including means for holding the melt in said first chamber.
 5. A furnace assembly for the growth of an uncoNtaminated arsenide compound crystal of predetermined composition, including: a gas-tight quartz housing, said housing having a first chamber adapted for the growth of crystals from a melt by the crystal pull technique, a second chamber and a neck interconnecting the two chambers along the vertical axis of said housing, said first chamber adapted to support a melt of the arsenide compound for crystal growth, said second chamber having a circular aperture coaxial with said axis and a port for the evacuation and admission of gas out of and into said housing, and said neck having a circular bore coaxial with said axis communicating with said chamber; a cylindrical rod passing through said aperture into said housing coaxial with said axis, said rod journaled into said bore and adapted for rotation about and vertical movement along said axis in said housing for the growth of a crystal from the melt; means external of said housing for rotating and vertically moving said rod for crystal growth; a predetermined amount of boron oxide in said housing, said boron oxide being stored as a solid at ambient temperature in said second chamber prior to the operation of said assembly, and said boron oxide sealing the space between said rod and the internal wall of said bore as a liquid at elevated temperatures; means external of said housing for heating and melting said boron oxide; means external of said housing for heating the melt of the arsenide compound; and means external of said housing for maintaining the crystal grown from the melt at a predetermined annealing temperature in said first chamber. 